Important parameters of particulate or powdered materials include particle sizes and particle size distribution. These parameters affect important physical and chemical properties of the materials, e.g., hiding power of pigments, gloss of paint films, packing properties of powders, sintering properties of metal powders, current leakage through dielectrics.
There are many known procedures for determining particle size and particle size distribution of a particulate sample. A common procedure for determining particle size distribution is to suspend the particulate sample in a liquid medium thereby forming a suspension. The suspension is subsequently subjected to either a gravitational or centrifugal force field. In response to the centrifugal or gravitational force exerted on the particles in suspension, the particles move through the liquid medium. This response to either gravitational or centrifugal force will hereinafter be referred to as settling. Observing the changes in powder concentration within the suspension with respect to time and depth of settling provides information which can be related to particle size distribution. In known centrifugal methods the time-dependent particle size concentrations are observed at a single fixed location relative to the settling chamber or tank.
U.S. Pat. No. 4,736,311 (Takeuchi et al.) discloses an apparatus for measuring particle size distribution which employs a centrifugal force field. In the apparatus there is provided a rotating disc to which a sample vessel is attached at the outer edge. By rotating the disc, a suspension placed within the sample vessel is subjected to a centrifugal force field. On one side of the rotating disc is positioned a fixed light source; on the other side there is positioned a light detector. When the rotation of the disc brings the sample vessel into line with the light source and light detector, a determination of particle concentration at a set point within the sample vessel can be made based on the intensity of light transmitted through the suspension and received by the light detector.
The apparatus is further provided with a centralized processing unit (CPU), random access memory unit (RAM), read only memory unit (ROM), timer, keyboard and display unit. The ROM unit stores equations which relate particle diameter to the time needed for the particle to reach the detecting point. Also, the ROM unit stores equations for correcting the effects of nonparallel particle movement on the observed particle concentrations in the case of either centripetal or centrifugal movement of the particles.
Another system for measuring particle size distribution by subjecting a suspension to centrifugal force is disclosed by U.S. Pat. No. 3,914,604 (Hornby et al.). In this system, the suspension is rotated within an annular chamber located at the periphery of a rotating disc-like member. A beam of electromagnetic radiation is passed through the annular chamber containing the rotating suspension. The particle size distribution is determined by measuring the absorption or scattering of the beam as it passes through the rotating suspension while the powder settles. Errors caused by changes in concentration due to radial motion and by an increase in centrifugal force with distance from the axis of revolution are mitigated by the choice of appropriate dimensions of the electromagnetic beam thickness, radius of the disc and sedimentation distances. The ratio of the thickness, d, of the electromagnetic radiation beam in a direction radial to the radial depth of the beam in the liquid from the radial inner surface thereof, r, is required to be not greater than 0.2. Similarly, the ratio of r to the radius from the center of the disc to the radial inner surface of the liquid, R, is required to be not greater than 0.1.
In the system, the disc is movable with respect to the fixed position of the beam source. However, during measurements of the absorbency or scattering of the beam, the position of the disc relative to the beam source is fixed. Also, as noted above, the choice of the detecting point, i.e., the point where the beam strikes the liquid sample, is governed by the relationship of d/r not being greater than 0.2.
U.S. Pat. No. 3,809,885 (Allen) discloses a device for measuring particle size distribution by subjecting the suspension to either a centrifugal or gravitational force field. The apparatus comprises a sedimentation tank, an isotope X-ray source on one side of the tank and a proportional counter operating as a detector on the opposite side of the tank. The X-ray source and counter are movable with respect to the tank whereby measurements of absorbency can be taken at different depths.
In one embodiment of the apparatus there is provided a disc-shaped tank which can be rotated. The suspension can thus be monitored with an X-ray beam at a known distance from the center. Also, the X-ray source and detector can be moved radially so that the single position of detection for taking the measurements for a particular distribution determination can be selected among a continuum from the center of the tank.